1. Field of Invention
The invention relates to a process for the production of electrolyte capacitors having a low equivalent series resistance and low residual current, electrolyte capacitors produced by this process and the use of such electrolyte capacitors.
2. Description of Related Art
A commercially available solid electrolyte capacitor as a rule comprises a porous metal electrode, an oxide layer on the metal surface, an electrically conductive solid which is incorporated into the porous structure, an outer electrode (contacting), such as e.g. a silver layer, and further electrical contacts and an encapsulation.
Examples of solid electrolyte capacitors are tantalum, aluminium, niobium and niobium oxide capacitors with charge transfer complexes, or pyrolusite or polymer solid electrolytes. The use of porous bodies has the advantage that because of the high surface area a very high capacitance density, i.e. a high electrical capacitance over a small space, can be achieved.
π-Conjugated polymers are particularly suitable as solid electrolytes because of their high electrical conductivity. π-Conjugated polymers are also called conductive polymers or synthetic metals. They are increasingly gaining economic importance, since polymers have advantages over metals in respect of processability, weight and targeted adjustment of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes), a particularly important polythiophene which is used industrially being poly-3,4-(ethylene-1,2-dioxy)thiophene, often also called poly(3,4-ethylenedioxythiophene), since it has a very high conductivity in its oxidized form.
Technical development in electronics increasingly requires solid electrolyte capacitors having very low equivalent series resistances (ESR). Reasons for this are, for example, falling logic voltages, a higher integration density and increasing cycle frequencies in integrated circuits. Furthermore, a low ESR also lowers energy consumption, which is particularly advantageous for mobile battery-operated uses. There is therefore the desire to reduce the ESR of solid electrolyte capacitors to as low a value as possible.
European Patent Specification EP-A-340 512 describes the preparation of a solid electrolyte from 3,4-ethylene-1,2-dioxythiophene and the use of its cationic polymers, prepared by oxidative polymerization, as a solid electrolyte in electrolyte capacitors. Poly(3,4-ethylenedioxythiophene), as a substitute for manganese dioxide or charge transfer complexes in solid electrolyte capacitors, lowers the equivalent series resistance of the capacitor due to the higher electrical conductivity, and improves the frequency properties.
A disadvantage of this and similar processes is that the conductive polymer is produced by polymerization in situ in the electrolyte capacitor. For this, the monomer, such as e.g. 3,4-ethylene-1,2-dioxythiophene, and oxidizing agent must be incorporated together or successively into the porous metal body in the presence of solvents and then polymerized. However, such a chemical reaction is undesirable in the production of electronic components, since it is very difficult to allow the chemical reaction to proceed always in an identical manner in millions of small porous components in order to produce capacitors of the same specification.
It is furthermore a disadvantage of in situ polymerization in the production of solid electrolytes for capacitors that the oxidizing agents can damage the dielectric (oxide layer) on the metal electrode. Transition metal salts, such as e.g. Fe(III) salts, are as a rule used as oxidizing agents. After the polymerization, not only the electrically conductive polymer but also the reduced metal salts, such as e.g. Fe(II) salts, remain in the electrode body as reaction products of the polymerization. An attempt can indeed be made to remove these salts by subsequent washing steps. However, this is expensive and is not achieved completely, i.e. residues of the metal salts still remain in the electrode body. As is known, transition metals in particular can damage the dielectric, so that the increased residual currents resulting from this significantly reduce the life of the capacitors or even render it impossible to use the capacitors under harsh conditions, such as high temperatures and/or high atmospheric humidity.
Furthermore, the production process for solid electrolyte capacitors is very expensive if an in situ polymerization is used: A polymerization process (impregnation, polymerization, washing) as a rules takes several hours, and under certain circumstances solvents which are an explosion hazard or toxic must be used in this, and very many polymerization processes are required in order to produce a solid electrolyte.
A further disadvantage of chemical in situ processes for the production of solid electrolyte capacitors is that as a rule anions of the oxidizing agent or optionally other monomeric anions serve as counter-ions for the conductive polymer. Because of their small size, however, these are not bonded to the polymer in a sufficiently stable manner. As a result, diffusion of the counter-ions and therefore an increase in the equivalent series resistance (ESR) of the capacitor may occur, especially at elevated use temperatures of the capacitor. The alternative use of high molecular weight polymeric counter-ions in the chemical in situ polymerization does not lead to films which are sufficiently conductive and therefore does not lead to low ESR values.
In Japanese Patent Application JP-A 2001-102255, a layer of polyethylenedioxythiophene/polystyrenesulfonic acid is applied directly to the oxide film for protection of the oxide film and better adhesion of the solid electrolyte to the oxide film. The solid electrolyte is then applied to this layer by means of in situ polymerization. However, this method also has the disadvantage that an in situ polymerization is necessary in order to produce a capacitor having a low ESR.
A polymerization of monomers can also be carried out electrochemically in the absence of oxidizing agents. However, the electrochemical polymerization requires that a conductive film is first deposited on the insulating oxide layer of the metal electrode. An in situ polymerization with all the abovementioned disadvantages is then in turn required for this. Finally, this layer must be electrically contacted for each individual metal electrode. This contacting is very expensive in mass production and can damage the oxide layer. Furthermore, electrochemical deposition into the pores of the porous metal electrode is very difficult, since the deposition primarily takes place on the outside of the electrode body due to the course of the electrical potential.